Provide a feature matrix CodiMD vs HackMD

I really applaud your work on CodiMD, which is IMHO the absolute best free/ibre open source pad, but as a CodiMD user and potential customer of HackMD, I want to know the differences in features between the different products that you develop and sell.

For example, as of right now, it's not possible to know if the "Book Mode" feature is available in CodiMD. (spoiler: it's not, which is really a shame)

Please look into making a feature matrix, similar to what Gitlab does (see their [Pricing](https://about.gitlab.com/pricing/), [Gitlab.com](https://about.gitlab.com/pricing/gitlab-com/feature-comparison/) and [Gitlab self hosted](https://about.gitlab.com/pricing/self-managed/feature-comparison/) feature pages).

Here a simple two-column comparison between CodiMD and HackMD would to the job.